Predictive Value of Semi-Quantitative GeneXpert Categories for Microbiological Outcomes in Pulmonary Tuberculosis

Background: Molecular testing has improved pulmonary tuberculosis (PTB) diagnosis, but the clinical interpretation of semi-quantitative GeneXpert results-particularly at low bacillary loads-remains uncertain. Methods: This retrospective study included 167 patients with positive GeneXpert results evaluated at a tertiary pneumology hospital between January and December 2024. Patients were stratified by culture status. Associations between semi-quantitative GeneXpert categories and smear positivity, culture confirmation, and time to culture positivity were evaluated using logistic regression, ROC analysis, and Cox proportional hazards models. Results: Increasing GeneXpert categories were associated in a graded association with microbiological positivity. Compared with "Very Low", the odds of smear positivity were higher for "Medium" (OR 36.00, 95% CI 6.49-199.65) and "High" results (OR 328.50, 95% CI 43.29-2492.98). The probability of culture confirmation increased stepwise (0.60 for "Very Low", 0.79 for "Low", 0.93 for "Medium", and 0.92 for "High"; AUC 0.70), indicating that bacillary load is only one of several determinants of culture positivity. Prior tuberculosis and underweight status were associated with "Very Low" results, while cavitary disease was associated with higher categories. Higher GeneXpert categories were also associated with shorter time to culture positivity. Conclusions: Semi-quantitative GeneXpert categories provide clinically relevant information in PTB. "Medium" and "High" results were usually associated with microbiological positivity, whereas "Very Low" results were less reliable and required cautious interpretation. These categories may support early clinical decision-making while culture results are pending.